Changes in the secretion of inhibin and steroid hormones during induced follicular atresia after hypophysectomy in the rat

Sequential changes in the function of antral follicles during the period of follicular atresia were investigated after hypophysectomy (Hypox) at 1100 hr on proestrus. Within 6 hours after Hypox, concentrations of progesterone (P), testosterone (T) and estradiol-17 beta (E) decreased abruptly in ovarian venous plasma (OVP) and follicles showed a reduced ability to ovulate. Six hours after Hypox, ovulation was still induced by human chorionic gonadotropin (hCG) in all animals but with significantly fewer number of oocytes compared to the group given hCG at 1100 hr on the day of proestrus. Nine hours after Hypox, several granulosa cells of all large follicles (greater than 400 microns in diameter) exhibited morphological signs of atresia. Twelve hours after Hypox, all large and medium sized (200-400 microns in diameter) follicles showed advanced stages of atresia and almost all follicles failed to ovulate in response to hCG. Inhibin activity in OVP declined more slowly compared to the profiles of steroid hormones and 53% of the initial inhibin activity was still maintained at 18 hours after the operation. Inhibin activity further decreased to 7% of the initial level at 24 hours and was undetectable by 48 hours after Hypox. These results suggest that fully developed Graafian follicles gradually lose their ability to secrete inhibin in contrast to the rapid decrease in secretion of steroid hormone during the process of atresia.